Polyacrylamide gel electrophoresis in presence of nonionic detergents reveals tissue-specific variation in the histones, the major chromosomal proteins. Since this was unexpected, a careful characterization of the newly resolved components is necessary. We are studying in detail the relationship between the evolution of tissue- specific histone complements and functional changes in the course of cell differentiation in the different cell types of the hemopoetic system. This study will provide the necessary control for comparing neoplastic cells with normal cells of comparable type and degree of differentiation. Tissue-specific histones are being isolated and used for producing specific antibodies necessary for isolating newly synthesized histones in the form in which they bind to DNA in vivo. Such histone transport forms will allow a more detailed investigation of gene specific binding of histones and of chromosome assembly in general.